1. Field of the Invention
The present invention relates to a transparent conductive film and a method of manufacturing the transparent conductive film.
2. Description of the Related Art
In detail, the present invention relates to a method of manufacturing a transparent conductive film formed on a heat-resistant inorganic substrate made of such as glass or ceramic, or a resin substrate made of such as a plastic film by using a coating method (a wet coating method) with heating at a low temperature lower than 300° C., in particular, at 100° C. to 250° C., the transparent conductive film having both of excellent transparency and high conductivity and also being excellent in film strength and resistance stability, and to the transparent conductive film obtained by the method of manufacturing the transparent conductive film. Furthermore, the present invention relates to an element and a transparent conductive substrate using the transparent conductive film, and to a device using the transparent conductive substrate.
As a material for forming a transparent conductive film for use in a transparent electrode for a display element such as a liquid-crystal display, electroluminescent display and a plasma display, in a touch panel, in a transparent electrode such as a solar panel, or for functional coating for such as reflecting heat rays, shielding electromagnetic waves, preventing charging and defogging, a tin-doped indium oxide (Indium Tin Oxide, which may be hereinafter referred to as “ITO”), which is a conductive oxide, is known.
As methods of manufacturing this transparent conductive film made of ITO (ITO film), physical methodologies are widely used, such as a vacuum deposition method, a sputtering method, and a chemical vapor deposition method. By these methods, a uniform ITO transparent conductive film being excellent in transparency and conductivity can be formed on a substrate.
However, a film forming apparatus to be used in these methods takes a vacuum container as a base, which is very expensive. Also, the component gas pressure in the manufacturing apparatus is required to be precisely controlled for each substrate film formation, thereby posing a problem in manufacturing cost and mass producibility.
As a manufacturing method to solve these problems, a method has been studied in which a coating liquid for forming transparent conductive film obtained by dissolving an indium compound and a tin compound in a solvent is used for coating a substrate (this method may be hereinafter referred to as a “coating method” or a “wet coating method”).
In this coating method, a transparent conductive film (ITO film) is formed with a simple manufacturing process of coating of a substrate with the coating liquid for forming transparent conductive film, drying, and baking. Known methods of coating of the substrate with the coating liquid include an inkjet printing method, a screen printing method, a gravure printing method, an offset printing method, a flexor printing method, a dispenser printing method, a slit coat method, a die coat method, a doctor blade coat method, a wire bar coat method, a spin coat method, a dip coat method, and a spray coat method.
As coating liquids for use in these coating methods, various coating liquids containing an indium compound and a tin compound have been conventionally developed. For example, an mixture of indium nitrate and alkyl tin nitrate containing halogen ions or a carboxyl group (for example, refer to Japanese Unexamined Patent Application Publication No. 57-138708), a mixture of an organic indium compound and an organic tin compound containing an alkoxyl group or the like (for example, refer to Japanese Unexamined Patent Application Publication No. 61-26679), a mixture of indium nitrate and an organic tin compound (for example, refer to Japanese Unexamined Patent Application Publication No. 4-255768), an inorganic compound mixture of indium nitrate, tin nitrate, and others (for example, refer to Japanese Unexamined Patent Application Publication No. 57-36714), a mixture of an organic indium nitrate such as dicarboxylate indium nitrate and an organic tin nitrate such as alkyl tin nitrate (for example, refer to Japanese Unexamined Patent Application Publication No. 57-212268), and an organic compound mixture solution made of an organic indium complex and a tin complex with coordination of acetylacetone (for example, refer to Japanese Examined Patent Application Publication No. 63-25448, Japanese Examined Patent Application Publication No. 2-20706, and Japanese Examined Patent Application Publication No. 63-19046) are disclosed.
In most of these conventionally-known coating liquids, a nitrate of indium or tin, an organic or inorganic compound made of a halide, an organometallic compound such as a metal alkoxide, and others are used.
However, since the coating liquid using a nitrate or a halide generates corrosive gas such as a nitrogen oxide or chlorine at the time of baking, there is a problem of causing corrosion of facilities and environmental pollution. As for the coating liquid using a metal alkoxide, the material is prone to hydrolytic degradation, thereby posing a problem in stability of the coating liquid. Moreover, most of the coating liquids using an organometallic compound described in the patent documents described above has poor wettability with respect to a substrate, and there is also a problem in which a non-uniform film tends to be formed.
To get around this, as an improved coating liquid with these problems mitigated, a coating liquid for forming transparent conductive film containing indium acetylacetonate (standard nomenclature: tris(acetylacetonato)indium: In(C5H7O2)3), tin acetylacetonate (standard nomenclature: di-n-butyl bis(2,4-pentanedionato) tin: [Sn(C4H9)2(C5H7O2)2]), hydroxypropylcellulose, alkylphenol and/or alkenylphenol, and dibasic acid ester and/or benzyl acetate is disclosed (for example, refer to Japanese Unexamined Patent Application Publication No. 6-203658).
In this coating liquid, with hydroxypropylcellulose being contained in a mixture solution of indium acetylacetonate and tin acetylacetonate, wettability of the coating liquid with respect to the substrate is improved. At the same time, the viscosity of the coating liquid is adjusted based on the content of hydroxypropylcellulose, which is a viscosity-adjusting agent, thereby making it possible to adopt various coating methods such as spin coating, spray coating, dip coating, screen printing, and wire bar coating.
Also, as an improved coating liquid for spin coating, a coating liquid for forming transparent conductive film is suggested (for example, refer to Japanese Unexamined Patent Application Publication No. 6-325637) containing an organic indium compound such as indium acetylacetonate or indium octylate, an organic tin such as tin acetylacetonate or tin octylate, and an organic solvent, in which as the organic solvent, an acetylacetone solution with alkylphenol and/or alkenylphenol dissolved therein or the acetylacetone solution with alkylphenol and/or alkenylphenol dissolved therein being diluted with alcohol is used.
This coating liquid has a low viscosity, and can be used not only in spin coating but also spray coating and dip coating.
Furthermore, for the purpose of improving performance of a transparent conductive film to be obtained, irradiation of ultraviolet rays has been tried to be combined with a simple film forming process of coating of a substrate with a coating liquid for forming transparent conductive film, drying, and baking. Examples of methods suggested include a method (refer to Japanese Examined Patent Application Publication No. 60-19610) in which at the time of coating with a coating liquid for forming transparent conductive film, the film is irradiated with ultraviolet rays from a high-pressure mercury lamp or a metal halide lamp to obtain a uniform and dense dried coating film and then the dried coating film is baked at a high temperature on the order of 500° C. and a method (refer to Japanese Unexamined Patent Application Publication No. 63-314714) in which the transparent conductive film obtained by baking at a high temperature equal to or higher than 500° C. (thermal decomposition) is irradiated with ultraviolet rays.
However, even with these methods, the resistance of the film is not sufficiently decreased. Also, the resistance once decreased with irradiation of the transparent conductive film obtained by baking with ultraviolet rays tends to be increased again due to storage in the atmosphere.
Thus, in order to obtain a transparent conductive film with low resistance and excellent in resistance stability, a method is suggested (refer to Japanese Unexamined Patent Application Publication No. 11-60278) in which a substrate is coated with a coating liquid for forming transparent conductive film containing an indium compound and a tin compound, then, during or after drying by preliminary heating at a temperature equal to or lower than 300° C., irradiation with ultraviolet rays having a wavelength equal to or smaller than 200 nm using, for example, a low-pressure mercury lamp is performed, and baking is further performed at a temperature equal to or higher than 300° C. (preferably, equal to or higher than 400° C.) under a non-oxidizing atmosphere.
Also a method is disclosed (refer to Japanese Unexamined Patent Application Publication No. 2001-106567) in which after a transparent substrate is coated with a coating liquid for forming transparent conductive film and is dried, it is baked in an oxygen atmosphere and is further baked in vacuum to obtain a transparent conductive film. According to this, by baking in the oxygen atmosphere, the baking temperature can be decreased to 180° C. Also, with irradiation with ultraviolet rays of a mercury lamp during baking in the oxygen atmosphere, decrease in resistance can be achieved. However, the wavelength of ultraviolet rays indicated by “ultraviolet rays of the mercury lamp” is not clear. Moreover, grounds for allowing the baking temperature to be decreased to 180° C. or specific examples are not shown. In addition, film characteristics (transmittance and resistance value) of the transparent conductive film obtained at the baking temperature of 180° C. are not described at all.
Therefore, this method seems to have a problem in actual use and also have many problems in practicability.
Still further, as a method of obtaining a metal oxide fine-particle film made of TiO2 fine particles, ITO fine particles, or the like with heating at a low temperature, a method is suggested (refer to Japanese Unexamined Patent Application Publication No. 2003-308893) in which plasma processing is performed on a metal oxide fine-particle containing coating layer obtained by coating a substrate with a coating liquid containing metal oxide fine particles and a binder and drying, thereby removing the binder.
However, this method has an object of obtaining a porous film with a large porosity by using fine particles already becoming a metal oxide as a filler of the coating liquid, and does not aim at forming a dense film to achieve an improvement in characteristics such as transparency, conductivity, film strength, and resistance stability of the transparent conductive film.
Meanwhile, a conductive oxide film having a conductive oxide such as indium oxide as a main component has been used for a transparent electrode typified by the ITO film described above, and additionally in recent years, has attracted attention as a channel active layer of a thin-film transistor and has been actively studied.
As a channel active layer of a field-effect thin-film transistor (TFT), a thin film made of amorphous silicon or the like formed on a glass substrate has been generally used so far. However, amorphous silicon has a low carrier mobility and does not sufficiently have characteristics as a thin-film transistor element.
Thus, many suggestions have been made in which various conductive oxide films are used as a channel active layer to improve the characteristics of a thin-film transistor element. Disclosed examples include a transparent thin-film transistor in which a transparent conductive oxide polycrystalline thin film using ZnO as a main component is used as a channel layer (refer to Japanese Unexamined Patent Application Publication No. 2002-76356 and Japanese Unexamined Patent Application Publication No. 2001-244464), a thin-film transistor in which an In—Ga—Zn—O-based transparent amorphous oxide semiconductor film (a-IGZO) is used as a channel active layer (refer to K. Nomura et. al., Nature, 2004, Vol. 432, pp. 488-492), a thin-film transistor in which a Ga—Zn—O-based or Ga—Sn—O-based non-monocrystalline oxide semiconductor is used as an active layer (refer to Japanese Unexamined Patent Application Publication No. 2007-123698), a thin-film transistor in which an In—Ga—Zn—O-based (InGaZnO4) non-monocrystalline oxide semiconductor is used as a channel active layer (refer to Japanese Unexamined Patent Application Publication No. 2007-123700), a thin-film transistor in which an In—Ga—Zn—Sn—O-based (InGaZnO4 doped with Sn) non-monocrystalline oxide semiconductor is used as a channel active layer (refer to Japanese Unexamined Patent Application Publication No. 2007-123699), and a thin-film transistor in which an amorphous oxide semiconductor made of any one or more of ZnO, SnO2, and In2O3 is used as a channel active layer (refer to Japanese Unexamined Patent Application Publication No. 2007-201366).
However, the conductive oxide films in the thin-film transistors described above (refer to Japanese Unexamined Patent Application Publication No. 2002-76356, No. 2007-123698, No. 2007-123700, No. 2007-123699, No. 2007-201366, and K. Nomura et. al., Nature, 2004, Vol. 432, pp. 488-492) are all formed by using a vapor phase method such as a sputtering method, a pulsed laser deposition method (a PLD method), or an electron beam deposition method, and not by using a coating method.
Moreover, in Japanese Unexamined Patent Application Publication No. 2001-244464, a coating liquid with zinc acetate being suspended in isopropanol is used to form a channel active layer made of ZnO, and after coating with the coating liquid, baking at a high temperature of 600° C. to 900° C. in air or in an oxygen atmosphere is required.
As described above, it is difficult to obtain a high-quality conductive oxide film (an oxide semiconductor film) suitable for a channel active layer of a thin-film transistor with a coating method using baking at a low temperature lower than 300° C.